THE 1883 AND LATE-PREHISTORIC ERUPTIONS OF AUGUSTINE VOLCANO, ALASKA

The eruptive history of Augustine volcano has been characterized by cy cles of growth and destruction of the volcano. Repeated failure of 5-1 0% of the edifice has produced mobile debris avalanches that reached t he sea on all sides. High lava extrusion rates rapidly restore the vol cano to its pre-failure configuration. This equilibrium between constr uctive and destructive processes has resulted in a relatively low lava -dome complex surrounded by an apron of volcaniclastic debris three ti mes the volume of the dome complex. The most recent edifice collapse o ccurred in 1883, producing the 0.3 km(3) Purr Point debris-avalanche d eposit. Three major slide blocks extended the shoreline up to 2 km and produced a tsunami that swept across Cook Inlet. Hummock morphologies change from a proximal radial orientation to a dominantly transverse alignment reflecting deceleration and compression of the avalanches as they enter the sea. The breached crater formed by collapse was then l argely filled by a 0.09 km(3) lava dome and a 0.04 km(3) lava flow tra velled down the north flank. Lithic block-and-ash flows and pyroclasti c surges reached the coast. The Burr Point avalanche deposit partially overlaps the Rocky Point debris-avalanche deposit to the west that wa s probably emplaced 200-400 years B.P. A major collapse event at ca. 1 540 +/- 110 A.D. produced the West Island debris avalanche, ending a p eriod of expansion of the western side of the island. An accompanying lateral blast overtopped the avalanche, covering a 40 degrees sector o f the west flank. The plinian tephra layer B may also have been erupte d at the time of the West Island eruption. Today Augustine volcano has rebuilt itself to a size similar to that which preceded the last edif ice failure in 1883. The frequency of past collapses (three in the las t 500 years) suggests that summit collapse is a possibility during any future eruption. The next major collapse is expected to involve 0.1-0 .5 km(3) of the summit; the ensuing debris avalanche would likely reac h the coast, producing a tsunami that could impact populated areas of the Kenai Peninsula. The largest tsunami would result from collapse in directions other than to the north or west. Tsunami magnitude is cont ingent on failure volume, direction and timing with respect to tides.